Certain catalysts containing cyclopentadienyl ligands, rare earth metals, and electron donors are known to polymerize olefin monomers. For example, U.S. Pat. No. 5,109,085 and Pettijohn et al., "Rare Earth Metallocene Polymerization Catalysts," a paper presented at Metcon '93 in Houston, Tex. (May 1993), describe olefin polymerization catalyst systems comprising a catalyst having the formula Cp.sub.n MX.sub.4-n.M'L.sub.x, wherein Cp is a cyclopentadienyl ligand, M is a metal having an atomic number in the range of 58 to 71, M' is an alkali metal, L is an electron donor ligand, and X is a halogen, along with a cocatalyst comprising an alkali or alkaline earth metal alkyl. These references describe the use of such catalyst systems in solution or slurry phase polymerization to generate olefin polymers of narrow molecular weight distribution.
Although cyclopentadienyl ligand/rare earth metal catalysts have demonstrated good polymerization activity, catalysts of this type are highly electrophilic and undergo destructive C--H activation, in which the rare earth metal center of the catalyst attacks the cyclopentadienyl ligand or the allylic hydrogen of a comonomer. To prevent this, workers in the art typically have loaded the cyclopentadienyl ligand with sterically bulky groups. For instance, Olonde et al., "Ethylene Polymerization on Neodymium-Metallocene Based Catalysts," an abstract of a presentation at the "Rare Earths '92 in Kyoto" Conference (Kyoto, Japan) describes the use of Cp*.sub.2 NdCl.sub.2 Li(OEt.sub.2).sub.2 with LiR or MgR.sub.2 as a catalyst composition for ethylene polymerization, in which Cp* is pentamethylcyclopentadienyl. However, use of bulky ligands like pentamethylcyclopentadienyl on such catalysts tends to lessen their ability to incorporate comonomers effectively.
Paolucci et al., J. Organomet. Chem., 471:97-104 (1994) describes the synthesis of certain bis(cyclopentadienyl)lanthanoid chlorides in which the cyclopentadienyl ligands are linked by a 2,6-dimethylenepyridyl group. The publication states that such compounds "may have catalytic applications (olefin hydrogenation or polymerization)." However, no further information is given on these applications, such as appropriate cocatalysts or reaction conditions.
It has now been discovered that olefin polymers may be produced using a catalyst composition comprising: a) a rare earth metal/cycloalkadienyl ligand catalyst in which an electron donor residue is attached directly to the cycloalkadienyl ligand or is attached to a bridging group connecting two cycloalkadienyl ligands, and b) an activating cocatalyst containing a Group I, II, or IIIA metal. The above catalyst is advantageously less electrophilic and therefore less prone to C--H activation than conventional cyclopentadienyl ligand/rare earth metal catalysts, yet allows comonomers sufficient access to the active metal center.